The electrophotographic process used in copying machines involves first electrically charging the surface of a photoconductor uniformly, projecting an image to the photoconductor from an optical system for forming a latent image on the photoconductor while allowing charges to be removed from the portion of the photoconductor that is exposed to light, followed by toner application and transfer of the toner image to paper. For uniformly charging the photoconductor surface to a desired potential, most conventional electrophotographic machines such as copying machines use a corona discharge device having a wire electrode and a shield electrode. The corona charging process, however, suffers from several problems including (1) generation of ozone or the like as a result of corona discharge, (2) a high voltage of 4 to 8 kV applied to provide a high potential of 500 to 700 V on the photoconductor, (3) low charging efficiency in that only a few percents of the corona current is utilized in charging, and (4) contamination of the wire electrode with dust and debris.
In order to eliminate these problems, a contact charging method was proposed in which an charger member is contacted with an object to be charged for electrically charging the object without using a corona discharge device. The prior art method falls in the concept of contact charging in that electric charging is conducted with the charger member and the object to be charged held in contact, but exactly speaking, relies on the mechanism that the object to be charged is charged by effecting air discharge through a fine gap between the charger member and the object to be charged. Therefore, the prior art contact charging method could reduce ozone generation as compared with the use of a corona discharge device, but could not fully suppress ozone generation. The charging method essentially relying on air discharge undesirably requires an extremely high charging onset voltage of several hundreds of volts in accordance with Paschen's law relating to air discharge across a narrow gap. We found that the charging onset voltage or charging threshold was often as high as 600 to 750 V and a high voltage of -1300 to -1500 V should be applied to provide a charging potential of -600 V, for example.
The conventional contact charging method sometimes applies a DC voltage having an AC voltage overlapped in order to maintain the charge potential uniform. This undesirably produces boisterous high-frequency noises due to air discharge.
Known charger members used in the conventional contact charging method include rollers of conductive rubber having carbon or other conductive particles dispersed therein, and such rollers covered with nylon, or the like. These charger members are given a necessary conductivity to continuously charge positive or negative an object to be charged. In the case of contact charging, however, consistent charging is not always achieved even if the charger member has a predetermined conductivity. For charger members having the same conductivity, for example, images bearing black peppers and fogs due to uneven charging appear with some members, but not with other members. This is a problem inherent to the contact method, not encountered in the corona discharge system. In addition, heretofore proposed charger members of natural rubber, butyl rubber, epichlorohydrin, silicone rubber or the like include many unknown factors in their behavior and are insufficient in charging performance and stability.